I'm guessing none of the SIMS I use in the EU are going to get me good rates in Vietnam. I'll be there for one week and in Hong Kong for 12 hours. Recommendations appreciated.

If you know someone who would add a line to their acct. on T-Mobile U.S., temporarily for $10/mo. - you would have unlimited text and data and 20¢./min. calling. Vietnam is one of their 120 "free" roaming countries. I did that for a cousin who went to Europe.

Make use of T-M's UMA/wifi free calling from any place in the world with access to wifi. I use an LG G6, wife an S7)
A/o Oct 20, 2013 no need for intl prepaid as T-Mobile U.S. includes voice roaming at 20¢/min (in and out)., unlimited text (in and out), and unlimited data in 140+ countries.

My Plan -[6 lines] U.S. T-Mobile unlimited minutes (incoming and outgoing), unlimited text, fast data on each line. that $145/mo. total! . (In U.S. no surcharge for calling a cell.) If a line exceeds 2G of data in a month, pay $10 more for that line. [That only happens a couple times/year.